Fly sheets with UV protection in the summer, a blanket in winter, keeping clipped means less likely to bleach, , a coat shine supplement (smartpak makes one especially for blacks if you don't show), healthy haircare or quic screen sunscreen or a fly spray with a sunscreen in it. If something bleaches still, shapely a makes black (and every other color) spray for touch ups. Try not to use liniments or alcohol excessively because it will dry out the hair and make it look duller. Lastly, both Gallop and Quic make shampoo for dark horses.
Lots and lots of good grooming. Also, remember a lot of good shine and looks come from the inside, so quality feed with lots of fat.
Keeping them covered in the sun helps. I don't like spraying them with lots of products, so I don't know how much the sunscreens and such help. I also rarely bathe my black/brown horse (the Quick Black does not work!). He has a lovely coat with a high shine with little more than good grooming, keeping him out of the sun or covered up, and lots of good food.
Do realize that some seemingly black horses AREN'T black (mine is one of them), and will turn brown no matter what.
-use a coat supplement (I used cocosoya for other reasons but it made my horse's coat look fabulous)
-make sure to sun screen as much as possible and fly sheet with UV protection -add baby oil to baths (just a little or else there will be too much slip)
-if there is any white on the horse, make sure that gets as clean as possible.
-I know some people dye their horse's mane and tail black to help hide sun damage
-there are shampoos for making black, but I can't speak to their effectiveness
-Feeding boiled beetroot and washing with mayonnaise will quickly darken a coat and the mayonnaise will add a shine (according to some people)
Leave them in during the day and out at night -- most effective method.
Some black horses fade more than others, whatever you do. I have 2, and one fades about 10 shades lighter than the other on totally equal turnout/treatment (they are even in the same field).
As another note -- be careful what fly spray you use. Absorbine Ultrashield in the black bottle will fade my black horses to burnt light brown in a week, despite supposedly containing "sunscreen." I can't use it at all. I have had decent luck with TriTec14 not fading them.
Also, make sure you hose them off after you ride -- salt on their coats will fade them. And leave them IN the barn until they dry -- being out in the sun wet will exacerbate fade.